Air conditioning systems and equipment are widely used to achieve desirable indoor comfort levels for both temperature and relative humidity in residential, commercial, industrial, and office settings. It is becoming increasingly more desirable to increase the amount of fresh outside air delivered into an air conditioned space. The additional amount of outside air may be desirable for health reasons, such as to reduce the likelihood of so-called sick building syndrome. Moreover, proposed government standards require that current standards of 5 cubic feet per minute (CFM) of outside air per person be trebled to 15 CFM per person.
Unfortunately, it is likely to be difficult to deliver 15 CFM per person of outside air at a desired low relative humidity. A conventional air conditioner includes a condenser, an evaporator and a compressor for recirculating refrigerant through the condenser and evaporator. The evaporator, which is cooled by the evaporating refrigerant, cools the air but may also typically produce air that is essentially saturated with moisture. Because outside air typically contains a relatively large amount of moisture, requiring a greater flow rate of outside air creates an even greater difficulty in achieving a desirable humidity level in the conditioned air.
Lower relative humidity in air conditioned air is also desirable because it allows a higher thermostat set point while providing for the same level of human comfort. In addition, lower humidity levels in air supply ducts may reduce mold, bacteria growth, and allergic reactions. For example, the industrial organization American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) suggests that air entering air delivery ducts be no greater than 70% relative humidity.
Relative humidity is decreased by removing moisture from the air as is achieved by a conventional evaporator and by heating the air to increase its volume while maintaining a constant amount of water contained therein. Accordingly, electrical resistance heaters have been used to reheat conditioned air downstream from the evaporator to reduce the relative humidity of the air being delivered to the conditioned space. For example, U.S. Pat. No. 4,813,474 to Umezu discloses a conventional air conditioner including electric strip resistance heaters for reheating cooled air downstream from the evaporator, and wherein a controller calculates a difference between actual and desired temperature and humidity levels and operates the apparatus accordingly. Unfortunately, conventional electric resistance heaters, although simple to install and operate, consume a relatively large amount of energy. Moreover, in certain jurisdictions, such as the state of Florida, for example, electric reheat is proscribed by law in certain applications because of its increased energy consumption.
Other approaches have been attempted to obtain reheating of the air flow downstream of the evaporator yet prior to entering air delivery ducts. For example, U.S. Pat. No. 5,337,577 to Eirmann discloses an air conditioner including a pair of connected heat exchangers on the upstream and downstream sides of the evaporator through which water or some other fluid is pumped to provide reheat in a run-around configuration. Supplemental heat may be provided by heat recovered from the refrigeration process or by an alternative energy source, such as a gas or electric boiler, or water heater. See also U.S. Pat. Nos. 5,228,302 and 5,181,552 to Eirmann. Unfortunately, a run-around heat exchange system may result in an increased pressure drop of the air flow, requiring increased power consumption and thereby reducing the overall operating efficiency. In addition, the run-around configuration may not provide sufficient reheating to achieve a desired low humidity when using a large percentage of outside air.
U.S. Pat. No. 5,329,782 to Hyde discloses an air conditioner wherein a refrigerant pressure boosting pump is connected between an outlet of the condenser and a subcooling coil positioned adjacent the evaporator. The subcooling coil provides heat to the flow of inlet air, thereby decreasing its relative humidity. The extraction of heat from the liquid refrigerant also serves to increase the effective capacity of the compressor.
Along these lines, U.S. Pat. No. 5,265,433 to Beckwith discloses an air conditioner including a supplemental loop and reheat coil which delivers heat to incoming air via a heat exchanger coupled to the hot compressor exhaust line. A subcooling coil and supplemental loop are also used to reduce the temperature of liquid refrigerant from the condenser by 30.degree. F. or more. Both of the heat exchangers disclosed in the Beckwith patent are phase change type heat exchangers wherein an intermediate phase change material is used to transfer heat.
Similarly, American Heat Pipes, Inc. of Auburndale, Fla. has offered an air conditioner including a subcooling coil and desuperheat reheat coil positioned in the flow of inlet air. The subcooling coil is directly connected in the refrigerant path to the evaporator. The desuperheat reheat coil provides additional controlled reheating to meet low cooling load conditions. The desuperheat reheat coil is coupled to the compressor discharge line via a heat pipe heat exchanger. The heat pipe heat exchanger is a phase change heat exchanger including a sealed tube charged with a precise amount of refrigerant to undergo a phase change and thereby transfer heat between the compressor discharge line and the desuperheat reheat coil. Unfortunately, as described above, a phase change heat exchanger may have reduced efficiency, be more difficult to control, and be relatively complex to install and maintain.
Also relating to control of relative humidity, Worthington Air Products of Palm Harbor, Fla. has offered an air conditioner comprising a plurality of subcooling coils, downstream from the evaporator, and connected in parallel with one another and a bypass. Selective operation of respective solenoid valves controls the amount of subcooling of the liquid refrigerant and thereby also controls the amount of air reheating. Unfortunately, while better control of reheating may be obtained, the system is relatively complex to install and operate.